U.S. Pat. No. 4,331,719 (Danielson et al.) describes tinted, spontaneous opal glasses ranging in color from ivory to brown. The tint is imparted by incorporating up to one percent nickel oxide (NiO) into the base glass composition. The patent further discloses that these tinted opals are adapted to use as the core glass in a three-ply glass laminate.
A line of opal glass tableware, produced in accordance with the teachings of the Danielson et al. patent, is marketed by Corning Incorporated under the registered trademark CORNERSTONER.RTM.. This ware exhibits a beige tint that is defined in copending application Ser. No. 464,372 (Amundson, Jr. et al.), that application being assigned to the assignee of the present application. The tint is there defined with reference to the standard CIE system utilizing chromaticity coordinates x and y and the tristimulus value Y. Acceptable x and y coordinates fall within a polygon ABCDEFA in the drawing of the Amundson, Jr. et al. application.
Chromaticity values are measured under standard conditions, i.e., Illuminant C, with a Hunter Colorimeter. They are a measure of light that diffusely reflects off opaque surfaces. Because the values obtained are readily reproducible, they are commonly employed to facilitate comparisons and to establish specification limits.
The CORNERSTONER.RTM. product is produced by cutting and molding desired shapes from a laminated glass sheet. The process is described in U.S. Pat. No. 3,673,049 (Giffen et al.). It has been found particularly useful in producing dinnerware, with the exception of one item. The process does not lend itself to molding a closed-handle cup or mug. The provision of this item to complete a dinnerware set has been a continuing problem.
One solution proposed has been to mold the cup from a gob of a traditional opal glass. However, the resulting product does not have the strengthening capability inherent in the laminated sheet product. Thus, a separate processing step would be required to impart extra strength.
Accordingly, consideration has been given to utilizing a glass-ceramic material, that is, a glass capable of being essentially uniformly thermally crystallized.
Glass-ceramic materials having a predominant crystal phase of potassium fluorrichterite, but modified by the presence of at least 10% cristobalite, are disclosed in U.S. Pat. No. 4,608,348 (Beall et al.). These materials are described as exhibiting great toughness and a modulus of rupture (MOR) of at least 20,000 psi, and incurring a limited degree of sagging during crystallization. These properties make the materials particularly useful in producing molded articles, such as dinnerware.
In particular, this provides an answer to the problem of producing a closed-handle cup, or mug, for inclusion in the CORNERSET.RTM. set. However, to be successfully used in this manner, the glazed glass-ceramic cup or mug must provide an exact color match.